Circulating current free type high frequency soft switching pulsewidth modulated full bridge DC/DC converter

ABSTRACT

An improved soft switching topology of a full bridge PWM DC/DC converter is described. The new topology employs an energy recovery snubber to minimize a circulating current flowing through the transformer and switching devices. By using an energy recovery snubber instead of adding a tapped inductor and a saturable reactor to reduce RMS current stress, the converter achieves nearly zero current switching for the right leg of the full bridge circuit due to the minimized circulating current, and achieves zero voltage switching for the left leg of the full bridge circuit due to the reflected output current during the interval of left leg transition. The converter achieves soft switching for secondary side rectifier and freewheeling diode because at the turn-on time of the primary switching device, the energy recovery snubber provides a low impedance path.

FIELD OF THE INVENTION

This invention relates to a high frequency soft switching phase-shiftedfull bridge DC/DC converter free from a circulating current flowingthrough the transformer and switching devices. The DC/DC converter wouldbe applied to the main unit of the power supply in a battery charger ofa telecommunication system, etc.

BACKGROUND OF THE INVENTION

Recently, many new techniques for high-frequency conversion have beenproposed to reduce the voltage and current stress to the component, andthe switching losses in the traditional pulsewidth-modulated (PWM)converter. Among them, the phase-shifted full-bridge (FB)zero-voltage-switched PWM techniques (Dhaval B. Dalal, "A 500 KHzMulti-Output Converter with Zero Voltage Switching", APEC, 1990) aredeemed most desirable for many applications because this topologypermits all switching devices to operate under zero-voltage-switching(ZVS) by using circuit parasitic characteristics such as transformerleakage inductance and power device junction capacitance.

The conventional high frequency phase-shifted full bridge DC/DCconverter has a disadvantage that the circulating current flows throughtransformer and switching devices during the freewheeling interval. TheRMS current stress and conduction losses of transformer and switchingdevices are increased by this circulating current.

This invention solves these problems by attaching an energy recoverysnubber (ERS or ERS1) to the secondary side of the transformer in theDC/DC converter. The energy recovery snubber (ERS or ERS1) of thisinvention has three fast recovery diodes Ds₁, Ds₂ and DS₃, two resonantcapacitors Cs₁ and Cs₂, and a small resonant inductor Lr which can beignored because the transformer leakage inductance L_(l) is used insteadof inserting the resonant inductor Lr.

FIG. 1 shows a prior art full bridge DC/DC converter schematic circuitand relevant wave-forms thereof are shown in FIG. 2 and FIG. 3. Thecircuit includes parasitic elements such as body diodes D₁, D₂, D₃ andD₄, junction capacitance Cp across each switching device, leakageinductance L_(l), and magnetizing inductance Lm of the transformer. Inthe case of regular PWM control, shown in FIG. 2, until the time at t₀,the energy is delivered from the source to the load through switches Q₁and Q₂. When the switches Q₁ and Q₂ are turned off, the load current I₀flows through rectifiers D₅ and D₆ during the freewheeling interval t₀-t₁ or t₂ -t₃. Then, the transformer primary current (I₁ (t)) becomeszero.

The main problem with this operating sequence is that when all fourswitches are turned off (t_(o), t₂), the energy stored in the leakageinductance of the power transformer causes severe ringing with thejunction capacitances of the switching devices.

To minimize the parasitic ringing as shown in FIG. 3, the gate signalsfor switches Q₂ and Q₄ are delayed (phase-shifted) with respect to thoseof Q₁ and Q₃, so that during the time interval t₂ -t₃ and t₇ -t₈ whenthe secondary voltage is zero, one of the primary switches is alwaysleft on. This provides a low-impedance path for the current of thetransformer leakage inductance L_(l) to circulate, thus solving theproblem of the parasitic ringing associated with the conventional PWMcontrol hard-switching FB converter (FIGS. 1 and 2).

However, when switch Q₁ is turned off at time t₁ (switch Q₃ at time t₆),the primary current I₁, which is the sum of the reflected output currentnI_(o) and the transformer primary magnetizing current Im, circulatesthrough Q₂ and D₃ during freewheeling mode t₁ -t₃ and decrease with aslope of the following equation (1): ##EQU1## wherein n is a turns ratioof the transformer given as n=Ns/Np.

Due to this circulating current, RMS current stress, conduction lossesof the transformer and switching devices are increased. The overallefficiency is also reduced.

SUMMARY OF THE INVENTION

The present invention is based on the object of providing a full bridgePWM DC/DC converter which minimizes the commutating and circulatingcurrent flowing through the transformer and switching devices. Byapplying an energy recovery snubber (ERS or ERS1), the converterachieves nearly zero current switching for the right legs Q₂ and Q₄ ofthe converter and obtains zero voltage switching for the left legs Q₁and Q₃ of the converter.

Further, the converter achieves soft switching for the secondary siderectifiers D₅ 1, D₆ and D₇. A circuit analysis and experiment accordingto the invention are performed to verify the topology by implementing a7kw (12 VDC, 58 A), 30 kHz Insulated Gate Bipolar Transistor (IGBT)based experimental circuit.

This invention solves the above described problems--RMS current stressand conduction losses and so on associated with the conventional highfrequency DC/DC converter. A high frequency, soft switching Full BridgeDC/DC converter free from a circulating current includes an energyrecovery snubber attached to the secondary side of the transformer.

The energy recovery snubber (ERS or ERS1) of the present inventionrecovers the switching losses of the transformer secondary side to theload. It has three fast recovery diodes Ds₁, Ds₂, and Ds₃, twocapacitors Cs₁ and Cs₂, and a small resonant inductor Lr. The smallresonant inducter Lr may be ignored or removed because the transformerleakage inductance L_(l) may be used instead of inserting the smallresonant inducer Lr in to the circuit.

The energy stored in the snubber capacitors Cs₁, Cs₂ during conductionmode begins discharging when the transformer secondary voltage in thefreewheeling intervals becomes zero. Due to the discharging of thesnubber capacitors Cs₁ and Cs₂, the output rectifiers D₅ and D₆ arereverse biased and the secondary windings of the transformer are open.Both primary and secondary currents in the transformer become zero.

Only a low magnetizing current Im circulates through D₃ and Q₂ duringthe freewheeling interval. Thus, the RMS current for the transformer andswitches is considerably reduced during the freewheeling interval. Theoverall efficiency can be increased by the resultant lowered conductionlosses. Additionally, the converter achieves nearly zero currentswitching for the right legs Q₂ and Q₄ due to the minimized circulatingcurrent during a right leg transition interval, and achieveszero-voltage-switching ZVS for the left legs Q₁ and Q₃ due to thereflected output current (nI_(o) =It₁, n=Ns/Np) during a left legtransition interval.

The converter achieves soft switching for secondary side rectifiers D₅and D₆ and freewheeling diode D₇ because at the turn-on time of switchesQ₁, Q₂, and Q₃, Q₄, the energy recovery snubber provides a low impedancepath, for example the path of transformer→snubber capacitor Cs₁ →snubberdiode Ds₃ →a small resonant inductor Lr (which can be ignored)→snubbercapacitor Cs₂.

BRIEF DESCRIPTION OF THE DRAWINGS

The circulating current free type, high frequency, soft switching PWMFull Bridge DC/DC converter according to the present invention will bedescribed in detail below with reference to embodiments shown in theaccompanying drawings, in which:

FIG. 1 shows a schematic circuit of a prior art full bridge DC/DCconverter;

FIG. 2 shows the wave forms of the hard-switching full bridge converterwith regular PWM of FIG. 1;

FIG. 3 shows the wave forms of phase-shifted FB DC/DC converteraccording to FIG. 1;

FIG. 4 shows a circuit of a high frequency, soft switching phase-shiftedfull bridge DC/DC converter with energy recovery snubber (center-tappedtransformer) having a circulating current free type, according to thepresent invention;

FIG. 5 shows the principal operation wave forms of the FIG. 4 circuit;

FIGS. 6(a), 6(b), 6(c), 6(d), 6(e), 6(f) show circuit configurations forthe six operating modes of FIG. 4, with FIG. 6(a) showing Mode 0: Q₁ Q₂conducting and Cs₁, Cs₂ charging; FIG. 6(b) showing Mode 1: Q₁ Q₂conducting (powering mode); FIG. 6(c) showing Mode 2: the left legtransition; FIG. 6(d) showing Mode 3: the freewheeling mode with Cs₁,Cs₂ discharging; FIG. 6(e) showing Mode 4: the right leg transition;FIG. 6(f) showing Mode 5: Q₄, Q₃ conducting and Cs₁, CS₂ charging;

FIG. 7 shows a circuit of a phase-shifted full-bridge DC/DC converterwith energy recovery snubber (ERS1) according to the present invention(center-tapped transformer without a snubber inductor);

FIG. 8 shows a circuit of a phase-shifted full bridge DC/DC converterwith energy recovery snubber (ERS) according to the present invention(single-tapped transformer with the snubber inductor);

FIG. 9 shows a circuit of a phase-shifted full-bridge DC/DC converterwith energy recovery snubber (ERS1) according to the present invention(single-tapped transformer without a snubber inductor);

FIG. 10 shows a circuit of a half-bridge DC/DC converter with energyrecovery snubber (ERS1) according to the present invention(center-tapped transformer without a snubber inductor);

FIG. 11 shows a circuit of a half-bridge DC/DC converter with energyrecovery snubber (ERS1) according to the present invention(single-tapped transformer) without a snubber inductor;

FIGS. 12(a) and 12 (b) show wave forms of the conventional phase-shiftZVS FB DC/DC converter with RCD snubber ERS in FIGS. 1 and 3 whereinFIG. 12(a) depicts the voltage and current waveforms present on theprimary side of the transformer and FIG. 12(b) depicts the voltage andcurrent waveforms present on the secondary side of the transformer.

FIGS. 13(a) and 13(b) show wave forms of the phase- shifted FB DC/DCconverter with energy recovery snubber (ERS) in FIGS. 4 and 5 whereinFIG. 13(a) depicts the voltage and current waveforms present on theprimary side of the transformer and FIG. 13(b) depicts the voltage andcurrent waveforms present on the secondary side of the transformer;

FIGS. 14(a) and 14(b) show wave forms of the soft switching FB DC/DCconverter with energy recovery snubber (ERS) (without a resonantinductor) in FIG. 7 wherein FIG. 14(a) depicts the voltage and currentwaveforms present on the primary side of the transformer and FIG. 14(b)depicts the voltage and current waveforms present on the secondary sideof the transformer;

FIG. 15 shows the efficiency by comparing the invented DC/DC converterwith the conventional ZVS DC/DC converter wherein the Vin applied toeach converter is 280 VDC and Vout is 120 VDC.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention will be now described with reference to the accompanyingdrawings. FIGS. 4 and 5 show a circulating current free type highfrequency, soft switching, phase-shifted full bridge DC/DC converterthat applies an energy recovery snubber (ERS) to minimize circulatingcurrent and their principal wave forms. A detailed circuit configurationof the power stage follows. Four switches Q₁ -Q₄ form the full bridgewith the power transformer.

The secondary side of high frequency transformer is center-tapped (orsingle-tapped) with output rectifiers. output inductor L_(f) and outputcapacitor C_(o) are used to smooth output current I_(o) and outputvoltage V_(o). The energy recovery snubber has three fast recover diodesDs₁, Ds₂ and Ds3, two resonant capacitor Cs₁ and Cs₂, and a smallresonant inductor Lr. The small resonant inductor Lr can be ignoredbecause the transformer leakage inductance L_(l) is used instead ofinserting the small resonant inductor Lr. The energy recovery snubber isinserted between the transformer secondary side rectifier D₅ and D₆ andthe output inductor Lf to reduce the circulating current.

By using the energy recovery snubber instead of adding a tapped inductorand a saturable reactor to reduce RMS current stress such as describedin references(S. Hamada, Y. Maruyama, M. Nakaoka, "Saturable ReactorAssisted Soft-Switching Technique in PWM DC-DC Converters", PESC, 1992;and S. Hamada, M. Michihira, M. Nakaoka, "Using A Tapped Inductor forReducing Conduction Losses in a Soft Switching PWM DC-DC Converter",EPE, 1993), the converter can reduce the circulating current flowingduring freewheeling intervals t₃ -t₄ and t₈ -t₉.

As shown in FIG. 5, the energy stored in the snubber capacitors Cs₂ andCs₁ during conduction mode (t_(o) -t₂, t₅ -t₇) starts discharging whenthe transformer secondary voltage in the freewheeling intervals becomeszero. Due to the discharging of the snubber capacitor Cs₁ and Cs₂, therectifiers D₅ and D₆ are reverse-biased and the secondary windings ofthe transformer are opened. Therefore, both primary and secondarycurrents of the transformer become zero. Only a low magnetizing currentIm circulates through D₃ and Q₂ during freewheeling interval in mode 3,shown in FIG. 6.

Thus, the RMS current through the transformer and switches isconsiderably reduced in the freewheeling intervals t₃ -t₄ and t₈ -t₉.Hence, the converter achieves nearly zero current switching for theright legs Q₂ and Q₄ due to the minimized circulating current during theinterval of right leg transition t₄ -t₅, and achieves zero voltageswitching for the left legs Q₁ and Q₃ due to the reflected outputcurrent (nI_(o) =It₁, n=Ns/Np) during the interval of left legtransitions t₂ -t₃. Also, the converter achieves soft switching forsecondary side rectifiers D₅ and D₆ and freewheeling diode D₇ because atthe turn-on time of switches Q₁, Q₂ and Q₃, Q₄, the energy recoverysnubber provides a low impedance path such as: transformer→snubbercapacitor Cs₁ →snubber diode Ds₃ →a small resonant inductor Lr (whichcan be ignored in this application)→snubber capacitor Cs₂.

A snubber adopted in this invention recovers the switching losses to theload. As the diode Ds₂ is reverse-biased with zero initial conditionsfor the passive elements, the circuit operation of the inventedcirculating current free type, soft switching Full Bridge DC/DCconverter is described as follows.

Mode 0: The reflected primary current (It₁ /n) begins to flow to Cs₁, Lr(Lr can be ignored in this application) and Cs₂ providing a lowimpedance path through the transformer and rectifiers D₅ and D₆ when theswitches Q₁, and Q₂ are conducting during time (t_(o) -t₁). During thecharging process, if the voltage across Cs₂ becomes higher than Vt2before the current reversing, Ds₂ starts conducting and constitutes anew resonant circuit through Lr and Cs₁. However, since Ds₃ is stillreverse-biased, snubber capacitor Cs₁ and Cs₂ are charged up to thetransformer secondary voltage Vt₂ evenly and stay charged until mode 2.##EQU2## Also, the converter is transferring input power to thesecondary side through transformer, D₆, D₅ and L_(f).

Mode 1: After the snubber capacitors Cs₁ and Cs₂ are charged up to Vt₂at time t₁, the converter transfers only input power to the load. If itis assumed that the output voltage V_(o) is a constant voltage sourceduring this mode (energy transferring mode), the transformer primarycurrent is increased with the slope of equation (3). ##EQU3## whereinL_(l) is leakage inductance and L_(f) /n² is reflected output filterinductor.

Mode 2: Just before the time t₂, the switches Q₁ and Q₂, and rectifiersD₅ and D₆ are conducting. When the switch Q₁ is turned-off at the timet₂, energy stored in the snubber capacitors Cs₁ and Cs₂ beginsdischarging, and a circulating current which is the sum of the reflectedoutput current nI_(o) and a low magnetizing current Im charges thejunction capacitance C_(p) of Q₁ and discharges the junction capacitanceC_(p) of Q₃ during the time interval t₂ -t₃. ##EQU4## After the junctioncapacitance voltage V₁ (t) of Q₁ reaches input voltage Vin, body diodeD₃ conducts prior to turn-on of the switch Q₃. Therefore, the switchesQ₁ and Q₃ easily achieve zero voltage switching due to the reflectedoutput current nI_(o) during the interval of the left leg transition t₂-t₃.

Mode 3: Due to the discharging of the snubber capacitors, Cs₁ and Cs₂,the rectifiers D₅ and D₆ are reverse-biased and the secondary windingsof the transformer are opened. Therefore, both primary and secondarycurrents of the transformer become zero. Only a low magnetizing currentIm circulates through D₃ and Q₂. The current I_(o) in output inductorL_(f) freewheels first through Cs₁, Cs₂ and L_(f), and then freewheelsthrough freewheeling diode D₇ and output inductor L_(f) after snubbercapacitors Cs₁ and Cs₂ are completely discharged. Thus, the RMS currentfor the transformer and switches is considerably reduced in thefreewheeling interval t₃ -t₄.

Mode 4: When switch Q₂ is turned-off with nearly ZCS and ZVS at the endof freewheeling period t₄, the right leg transition starts and only alow magnetizing current Im flowing through the primary transformertransfers little energy to capacitors C_(p) of the switching devices Q₂and Q₄. The stray capacitance voltage V₂ (t) of Q₂ increases with aslope of equation (5). ##EQU5##

Mode 5: Switch Q₄ is also turned-on with ZCS at time t₅. Like Mode 0,the reflected primary current It₁ /n begins to flow to Cs₁, Lr (again Lrcan be ignored) and Cs₂ provide a low impedance path through transformerand rectifiers D₅ and D₆ and the input power is transferred to the load.

A circulating current free type, high frequency, soft switching DC/DCconverter, according to the invention, has as its main circuit thephase-shifted full bridge DC/DC converter consisting of main switchingdevices Q₁, Q₂, Q₃ and Q₄, center-tapped (or single-tapped as shown FIG.8) transformer, rectifiers D₅ and D₆, freewheeling diode D₇, outputfilter L_(f) and C_(o), and energy recovery snubber ERS. Thiscirculating free type full bridge DC/DC converter has conduction lossesfrom the switching devices, copper loss in the high frequencytransformer, and reverse recovery loss of rectifiers D₅ and D₆ alongwith freewheeling diode D₇, which should be reduced. The energy recoverysnubber attached between the transformer secondary side rectifiers D₅and D₆ and output inductor L_(f) consists of three fast recovery diodesDs₁, Ds₂ and Ds₃, two capacitors Cs₁ and Cs₂ and a small resonantinductor Lr (which can be ignored because the transformer leakageinductance L_(l) is useable instead of inserting a small resonantinductor Lr as shown FIG. 7).

The energy stored in the snubber capacitor Cs₁ and Cs₂ during conductionintervals t_(o) -t₂ and t₅ -t₇ starts discharging when the transformersecondary voltage during freewheeling intervals t₃ -t₄ and t₈ -t₉becomes zero. Due to the discharging of the snubber capacitors Cs₁ andCs₂, the output rectifiers D₅ and D₆ are reverse-biased and thesecondary windings of the transformer are opened. Both primary andsecondary currents of the transformer become zero. Only a lowmagnetizing current Im circulates through D₃ and Q₂ during freewheelinginterval t₃ -t₄ or D₁ and Q₄ in freewheeling interval t₈ -t₉.

The RMS current through the transformer and switches is considerablyreduced in the freewheeling intervals t₃ -t₄ and t₈ -t₉. Therefore,conduction loss of the switching devices and copper loss of the highfrequency transformer are reduced. Also, the converter achieves nearlyzero current switching for the right leg Q₂ and Q₄ due to the minimizedcirculating current during the interval of right leg transition, andachieves zero voltage switching for the left legs Q₁ and Q₃ due to thereflected output current (nI_(o) =It₁, n=Ns/Np) during the interval ofleft leg transition.

The converter achieves soft switching and reduces a reverse recoveryloss for secondary side rectifiers D₅ and D₆ along with freewheelingdiode D₇ because at the turn-on time of switches Q₁, Q₂ and Q₃, Q₄, theenergy recovery snubber ERS provides a low impedance path such astransformer→snubber capacitor Cs₁ →snubber diode Ds₃ →a small resonantinductor Lr (which can be ignored) →snubber capacitor Cs₂.

The circulating current free type, high frequency, phase-shifted fullbridge (or half bridge) DC/DC converter according to the invention isable to apply the energy recovery snubber ERS1 as shown in FIG. 7, FIG.9, FIG. 10 and FIG. 11 in which a small resonant inductor Lr is notincluded. Due to use of the energy recovery snubber ERS1, the inventedconverter reduces the conduction losses in the primary side becauseoutput rectifiers D₅ and D₆ are reverse-biased and the secondarywindings of the transformer TR are opened by blocking the voltage of thesnubber capacitors Cs₁ and Cs₂ during the discharging mode thereof.Also, the converter with energy recovery snubber ERS1 does not generatethe severe parasitic ringing or create diode reverse recovery loss inthe secondary side because the energy recovery snubber ERS1 provides alow impedance path such as transformer →snubber capacitor Cs₁ →snubberdiode Ds₃ →snubber capacitor Cs₂ during the conduction mode. However,the converter has a minor drawback wherein the secondary side voltageVt₂ is slightly increased as shown in the experimental wave-forms ofFIG. 14.

The circulating current free type, high frequency, phase-shifted fullbridge DC/DC converter of the invention has 6 operating modes in orderto achieve soft switching and the reduced conduction loss, and hasoperational wave-forms corresponding with the above modes as shown FIG.5 and FIG. 6.

The circulating current free type, high frequency, soft switching,phase-shifted full bridge DC/DC converter of this invention is able touse not only center-tapped transformer (FIG. 4, FIG. 7 and FIG. 10) butalso single-tapped transformer (FIG. 8, FIG. 9 and FIG. 11).

The half bridge DC/DC converter, as shown in FIG. 10 and FIG. 11, isconnect the energy recovery snubber ERS1 between the transformersecondary side rectifiers D₅ and D₆ and output inductor L_(f) to achievesoft switching and to reduce reverse recovery losses for main switchingdevices Q₁ and Q₂ secondary side rectifiers D₅ and D₆ and freewheelingdiode D₇.

According to the present invention, experimental results are shown as inthe followings: A 7Kw (120VDC, 58A), 30kHz IGBT based experimentalcircuit has been implemented to demonstrate the operation. Theparameters of the circuit are as follows:

Q₁ -Q₄ : IGBT (2MBI120L060, 600V, 200A)

D₁ -D₄ : Body diodes of IGBT

C_(p) : 14 nF (stray capacitance of IGBT)

Lm: 286 uH (magnetizing inductance of transformer)

L_(l) : 3.5 uH (leakage inductance of transformer)

n: Transformer turn ratio (n=Ns/Np=6/8=0.75)

Cs₁, Cs₂ : 0.2 uF (snubber capacitor)

Lr: 3 uH (snubber inductor)

Ds₁ -Ds₃ : snubber diode

D₅, D₆, D₇ : Rectifier, freewheeling diode

L_(f) : 500 uH (output inductor)

δt: 1.3 us (dead time).

FIG. 12, FIG. 13 and FIG. 14 show the voltage and current wave-form ofthe primary and secondary side of transformer in the converter with aRCD snubber (FIG. 1 and FIG. 3) and the converter with an energyrecovery snubber (FIG. 4, 5 and FIG. 7), respectively. Comparing FIG. 12with FIG. 13 and FIG. 14, it can be seen that by using an energyrecovery snubber ERS or ERS1, the circulating current I₁, decreasesnearly to zero and only a low magnetizing current Im is flowing duringthe freewheeling interval. Also, due to the reduced circulating current,soft switching is achieved without peak voltage and severe ringing froma light load to a full load.

FIG. 15 shows the measured efficiency of the proposed soft switchingDC/DC converter in comparison with the conventional ZVS DC/DC converter.The efficiency of the invented converter with an energy recovery snubbershows especially a low efficiency characteristics in a light load state(below 16 A) even though the circulating current is reduced during thefreewheeling interval. The reduced efficiency characteristics in thelight load state is due to the charging current flowing through thesnubber capacitors. However, when the load current is increased above16A, the efficiency of the invented converter with an energy recoverysnubber shows some improvement (2% -4%) over the conventional ZVS DC/DCconverter due to reduced circulating current and application of thenon-dissipated snubber.

The converter according to the present invention can minimize thecommutating and circulating current flowing from the transformer andswitching devices. By applying an energy recovery snubber, the converterachieves nearly zero current switching for the right legs Q₂ and Q₄ andachieves zero voltage switching for the left legs Q₁ and Q₃.

Further, the converter achieves soft switching for the secondary siderectification diodes D₇, D₅ and D₆. A circuit analysis and experimentare performed to verify the proposed topology by implementing a 7 kW(120VDC,58 A), 30 kHz IGBT based experimental circuit.

What I claim is:
 1. A DC/DC converter comprising:a DC/DC convertercircuit including:first and second switching transistors that areseries-connected together across terminals for receiving an inputvoltage; third and fourth switching transistors that areseries-connected to each other and that are connected in parallel acrosssaid first and second switching transistors; a transformer through whicha circulating current flows having a primary winding that is connectedbetween a junction of said first and second switching transistors and ajunction of said third and fourth switching transistors and a secondarywinding, said secondary winding having a center tap; first and secondrectifying diodes, each said rectifying diode having an anode attachedto a separate end of said secondary winding; a freewheeling diode havinga cathode connected to cathodes of said rectifying diodes, and an anodeconnected to said center tap of said secondary winding; an output filterconnected at one end to a cathode of said freewheeling diode; and anenergy recovery snubber connected between said rectifying diodes andsaid output filter, said energy recovery snubber including:a firstsnubber diode and a first snubber capacitor that are series connectedtogether and that are connected across said freewheeling diode whereinan anode of said first snubber diode is connected to an anode of saidfreewheeling diode, and one end of said first snubber capacitor isconnected to said cathode of said freewheeling diode; a second snubberdiode and a second snubber capacitor that are series connected togetherand that are connected across said freewheeling diode wherein a cathodeof said second snubber diode is connected to said cathode of saidfreewheeling diode, and one end of said second snubber capacitor isconnected to said anode of said freewheeling diode; a third snubberdiode connected in a forward biased orientation between a junction ofsaid first snubber diode and said first snubber capacitor and a junctionof said second snubber diode and said second snubber capacitor, so thatsaid snubber capacitors are charged during conduction modes anddischarged during freewheeling modes, said first and second rectifyingdiodes being reverse biased when said capacitors are discharged so thatsaid secondary winding is opened so that, in the freewheeling mode, thecirculating current flow through said secondary winding falls to zero soas to cause a reduction in current flow through said primary winding,said energy recovery snubber being free from a snubber inductor.
 2. TheDC/DC converter of claim 1, wherein the first and second snubbercapacitors have a value of 0.2 μ f.
 3. A half-bridge DC/DC convertercomprising:a DC/DC converter circuit including:first and second bridgecapacitors that are series-connected together across terminals forreceiving an input voltage; first and second switching transistors thatare series-connected to each other and that are connected in parallelacross said first and second bridge capacitors; a transformer throughwhich a circulating current flows having a primary winding that isconnected between a junction of said first and second bridge capacitorsand a junction of said first and second switching transistors and asecondary winding, said secondary winding having a single tap; first andsecond rectifying diodes, said first and second rectifying diodes havinganodes attached to a separate end of said secondary winding and cathodesconnected to each other; third and fourth rectifying diodes, said thirdand fourth rectifying diodes having cathodes attached to separate endsof said secondary winding, and anodes connected to each other; an outputfilter connected at one end to said cathodes of said first and secondrectifying diodes and at another end to said anodes of said third andfourth rectifying diodes; and an energy recovery snubber connectedbetween said rectifying diodes and said output filter, said energyrecovery snubber including:a first snubber diode and a first snubbercapacitor that are series connected together across said output filterwherein an anode of said first snubber diode is connected to said anodesof said third and fourth rectifying diodes, and one end of said firstsnubber capacitor is connected to the cathodes of said first and secondrectifying diodes; a second snubber diode and a second snubber capacitorthat are series connected together and that are connected across saidoutput filter wherein a cathode of said second snubber diode isconnected to said cathodes of said first and second rectifying diodes,and one end of said second snubber capacitor is connected to said anodesof said third and fourth rectifying diodes; and a third snubber diodeconnected in a forward biased orientation between a junction of saidfirst snubber diode and said first snubber capacitor and a junction ofsaid second snubber diode and said second snubber capacitor, said energyrecovery snubber being free from a snubber inductor and free from afreewheeling diode.
 4. The DC/DC converter of claim 3, wherein saidtransformer has a turn ratio of 0.75.
 5. The DC/DC converter of claim 3,wherein said first and second snubber capacitors have a value 0.2 μ f.6. A half-bridge DC/DC converter comprising:a DC/DC converter circuitincluding:first and second bridge capacitors that are series-connectedtogether across terminals for receiving an input voltage; first andsecond switching transistors that are series-connected to each other andthat are connected in parallel across said first and second bridgecapacitors; a transformer through which a circulating current flowshaving a primary winding that is connected between a junction of saidfirst and second switching transistors and a secondary winding, saidsecondary winding having a center tap; first and second rectifyingdiodes, each said rectifying diode having an anode attached to aseparate end of said secondary winding; a freewheeling diode having acathode connected to cathodes of said rectifying diodes, and an anodeconnected to said center tap of said secondary winding; an output filterconnected at one end to a cathode of said freewheeling diode; and anenergy recovery snubber connected between said rectifying diodes andsaid output filter, said energy recovery snubber including:a firstsnubber diode and a first snubber capacitor that are series connectedtogether and that are connected across said freewheeling diode whereinan anode of said first snubber diode is connected to an anode of saidfreewheeling diode, and one end of said first snubber capacitor isconnected to said cathode of said freewheeling diode; a second snubberdiode and a second snubber capacitor that are series connected togetherand that are connected across said freewheeling diode wherein a cathodeof said second snubber diode is connected to said cathode of saidfreewheeling diode, and one end of said second snubber capacitor isconnected to said anode of said freewheeling diode; a third snubberdiode connected in a forward biased orientation between a junction ofsaid first snubber diode and said first snubber capacitor and a junctionof said second snubber diode and said second snubber capacitor, so thatsaid snubber capacitors are charged during conduction modes anddischarged during freewheeling modes, said first and second rectifyingdiodes being reverse biased when said capacitors are discharged so thatsaid secondary winding is opened so that, in the freewheeling mode, thecirculating current flow through said secondary winding falls to zero soas to cause a reduction in current flow through said primary winding,said energy recovery snubber being free from a snubber inductor.
 7. TheDC/DC converter of claim 6, wherein the transformer has a turn ratio of0.75.
 8. The DC/DC converter of claim 6, wherein the first and secondsnubber capacitors have a value 0.2 μ f.